1. Field of the Invention
The present invention relates to an image output circuit, for use with television receivers and monitor equipment and so forth, for amplifying image signals and driving image-receiving tubes, and more particularly to an image output circuit for television receivers and the like employing a flat, color image-receiving tube for field-sequential displaying.
2. Description of Related Art
One adjustment carried out on television receivers and monitor equipment and the like is a white balance adjustment.
For example, in a color image-receiving tube, in order to show white, the levels of the emitted red, green and blue light must first be mixed evenly. However, there are differences in the light-emitting efficiency between each of the colors of the light-emitting bodies and differences between the electron guns corresponding to each color. If each color is then outputted as is at an equal electron beam power, the fluorescent materials for each of the colors red, green and blue do not generate light at the same level. As a result, a satisfactory white color will not be obtained, giving the display screen an overall off-color effect. At the image output circuit controlling the strength of the electron beam shone onto the fluorescent screen, white balance may be adjusted by varying the gain and adjusting the light generating level of each of the luminous bodies for the colors red, green and blue colors so that an appropriate white color may be obtained on the screen.
FIG. 1 is a circuit diagram showing an example of an image output circuit including a circuit for carrying out this kind of white balance adjustment. In this case, the so-called simultaneous primary color drive method is shown, where each cathode of the image-receiving tube drives a primary color signal corresponding to red, green and blue outputting three electron beams simultaneously.
In this drawing, first, Z indicates an image-receiving tube employing a cathode ray tube (hereinafter referred to as a "CRT") having a display screen with a body luminous with respect to the three colors red, green and blue, arranged in a stripe shape. Cathodes, KR, KG and KB, and grids GR, GG and GB corresponding to red, green and blue of the reception tube Z are also shown.
Further, Q1, Q2 and Q3 indicate transistors acting as amplifiers taking an input and amplifying each color source signal corresponding to red, green and blue generated by circuit parts of previous stages not shown in the drawings. The primary color signal R for red is connected to the base of transistor Q1. The collector is connected to the main power line VCC via the collector resistor R1 and is also connected to the cathode KR via a discharge protection resistor R4. The emitter is connected to the white balance adjuster 1.
Similarly, for transistor Q2, the signal G for the primary color green is connected to the base, the collector is connected to the main power line VCC via the collector resistor R2 and to the cathode KG via the discharge protection resistor R5, and the emitter is connected to the white balance adjuster 1. Further, the signal B for the primary color blue is provided at the base of the transistor Q3. The collector is connected to the power supply line VCC via the collector resistance R3 and to the cathode KB via the discharge protection resistor R6, and the emitter is connected to the white balance adjuster 1. Furthermore, it is also possible to use an amplifier apparatus consisting of amplifier circuits with operational amplifiers and the like in place of those with transistor elements.
Next, numeral 1, provided on the emitter side of the transistors Q1, Q2 and Q3, indicates the white balance adjuster for adjusting of the white balance. This white balance adjuster 1 comprises a drive adjuster 2 for adjusting the high band (light portions) gain of each of the colors red, green and blue (hereinafter referred to as drive adjustment), and a background adjuster 3 for adjusting dark parts of each of the colors red, green and blue, i.e., adjusting the gain (hereinafter referred to as background adjustment) of the black level portions.
Further, drive adjustment resistors RDR, RDG and RDB have been provided at the drive adjuster 2 so that the emitter-earth resistance of the transistors Q1, Q2 and Q3 can be varied as necessary. Arbitrary variation of these resistances varies the amplification factor of the transistors Q1, Q2 and Q3 and the gain of each of the red, green and blue color signals may therefore be adjusted.
Moreover, background resistors RBR, RBG and RBB are provided at the background adjustment part in parallel across the main power line VCC and earth so that the voltage dividing point may be varied. The voltage dividing point of the background adjustment resistor RBR is connected to the emitter of transistor Q1 via resistor R7. The voltage dividing point of the background adjustment resistor RBG is connected to the emitter of transistor Q2 via resistor R8 and the voltage dividing point of the background adjustment resistor RBB is connected to the emitter of transistor Q3 via resistor R9. As a result, the voltage ratio obtained by varying each of the background adjustment resistors RBR, RBG and RBB and the d.c. voltages set up by the resistors R7, R8 and R9 are superimposed across each of the emitter-earth junctions of the transistors Q1, Q2 and Q3. The black level may then be adjusted by varying this value.
In this kind of construction, each of the red, green and blue primary color signals inputted to the base of the transistors Q1, Q2 and Q3 is amplified taking the collector resistances R1, R2 and R3 as output resistances. Each of the primary color signals are then varied in synchronization with potentials applied to each of the cathodes KR, KG and KB via resistors R4, R5 and R6. A satisfactory white balance may then be obtained by adjusting the strength of the light and dark areas while, for example, a gray scale image or the like is displayed on the image-receiving tube Z so as to graduate from white to black. The adjustments are made by altering the gains of transistors Q1, Q2 and Q3 by adjusting each of the drive adjustment resistors RDR, RDG and RDB of drive adjuster 2 and each of the background adjustment resistors RBR, RBG and RBB of the background adjuster 3.
When a shallow, flat image-receiving tube is used as opposed to the so-called "straight type" of image-receiving tube described above, displaying a simultaneous color image becomes difficult. Because of this, a picture corresponding to each of the colors red, green and blue is displayed every one field and one frame is formed from three of these field pictures. It is then preferable to adopt a so-called "field-sequential system" where a single color image is expressed by after-image phenomena obtained using each field picture.
More specifically, the field images for the colors red, green and blue are obtained by placing a filter which allows only one of the colors red, green or blue to pass through in front of the image-receiving tube. Sequential changeover control is then exerted to ensure that only one of the colors red, green and blue is allowed to pass through per each one field.
In addition, it is also necessary to obtain a satisfactory white balance for the field-sequential system by adjusting the strength of each color of the red, green and blue field images which have been combined into groups comprising three fields. However, in the case of the field-sequential system, individual electron beams for each of the colors red, green and blue cannot be obtained in the way for the simultaneous system in FIG. 1 because the electron beams are outputted by a single electron gun. The white balance can therefore not be adjusted by setting up the image output gain for each of the colors red, green and blue beforehand.